Recently radio devices, including portable telephones, cordless telephones and radio sets, are constantly becoming smaller and lighter, and their use is spreading rapidly. For the high frequency circuits of these radio devices, filter elements are used as critical key devices.
Therefore corresponding to the demands of smaller size and light weight for radio devices, smaller and lighter filter elements are demanded. As a configuration to make a filter element smaller and lighter, a surface acoustic wave device is used.
When the surface acoustic wave device is used as a filter, a critical issue is decreasing the insertion loss and increasing the attenuation amount outside the pass band in order to allow necessary signals to pass and to remove unnecessary signals. Also as the number of components decreases for downsizing the radio device main body, characteristics demanded for the surface acoustic wave device are becoming critical.
The surface acoustic wave device in the configuration example in FIG. 1 includes an interdigital transducer (IDT) 1 and reflectors 2 and 3, which are formed by electrode finger patterns provided on the piezoelectric substrate 100.
In the example in FIG. 1, the interdigital transducer (IDT) 1 includes three individual IDTs (these are hereafter called drive electrodes 1-1, 1-2 1-3), and the drive electrodes 1-1 and 1-2 are connected in parallel to the input terminal IN. The drive electrode 1-3 is connected to the output terminal OUT. The drive electrodes 1-1, 1-2 and 1-3 and the input terminal IN, and the output terminal OUT are connected respectively by lead wires which are connected to the electrode pad 4.
In the example in FIG. 1, the reflectors 2 and 3, which are disposed on both sides of the interdigital transducer 1, constitute a conventional grating electrode.
In this configuration, one possible method for implementing a narrower band is increasing the number of electrode fingers of the interdigital transducer 1. This however increases insertion loss. Another method of implementing a narrower band is decreasing stop band width by decreasing the reflection coefficient of the reflection electrodes 2 and 3, but this also has a tendency to increase insertion loss accordingly.